1. Field of the Invention
This invention relates to clamps for lifting articles such as steel plates and more particularly to such clamps having a locking device to retain the clamp jaws in either an open or closed position.
2. Description of the Prior Art
U.S. Pat. No. 2,654,630 discloses a clamp, comprising a clamp body having a slot to receive an article to be lifted. A jaw is provided on each side of the slot adjacent the outer open end of the slot. The gripping jaw is movable between open and closed positions relative to the other jaw which is generally fixed. The movable jaw is operated by a shackle connected to the movable jaw by a force multipling linkage.
The clamp body includes a pair of spaced side plates. Mounted on the inside of the side plates is a pivotal locking plate operated by a handle located outside the side plate. A heavy tension spring connects the locking plate to the linkage which extends between the shackle and the movable jaw. By rotation of the locking handle to a closed position, the spring exerts a heavy closing force on the movable jaw. At the same time however, the spring tends to cant the pivot pin connecting the locking plate to the handle to a position inclined relative to its normal axis. This causes a heavy operating action which makes the lever difficult to operate.
It has been found that some clamps tend to permit a plate being lifted to engage a portion of the slot of the clamp in swinging back and forth such that the plate tends to "walk out" of the slot and its engagement with the clamp jaws.
Also in clamps, in their use, there could be a danger of the operating handle striking an object while the steel plate is being lifted which releases the handle to its open position, thus running the danger of the steel plate becoming released from the clamp during the lifting operation.